Direct the flat disk toward the light source, press the button for a couple of seconds, release, and it will hold the reading.
If the light source is bright enough (and indoors it often isn't bright enough), it will put you in the ballpark.
I find it useful sometimes to decide outdoors where there might be a combination of factors (overcast, shade, etc.) how much of a warming filter I might need, but with a little experience, you'll be able to read those situations without a meter.
Indoors, I find it hard to get a believable reading usually.